Jawabanmu

2014-02-18T14:29:11+07:00
U4 = a + (4-1)b = 15 --> a + 3b = 15 ==> a = 15 - 3b
U9 = a + (9-1)b = 30 --> a + 8b = 30
Misalkan pakai subtitusi =
a + 8b = 30
15 - 3b + 8b = 30
 5b = 15
b = 3
a + 8b = 30
a + 8.3 = 30
a = 30 - 24
a = 6
Sn = n/2 . (2a +(n-1)b)
S20 = 20/2 . (2.6 + (20-1)3)
= 10. (12 + 57)
= 10 . 69 = 690. Semoga saja benar :D

maksih bener kok :)
oke :D saling membantu ;)